The Cu interconnect in Si-LSI has been fabricated with a diffusion barrier in the ditch of SiOィイD22ィエD2 by the so-called damascene process. For the Cu interconnect technology, (1) a preferential orientation of Cu (111) plane is desired for suppressing an electromigration failure, and (2) an effective diffusion barrier is also desired as thin as possible. The diffusion barrier materials, which satisfy these issues, are examined in the present study. Transition metal of Nb or V, which is in the bcc structure, is applied as a diffusion barrier between Cu and SiOィイD22ィエD2. The results show that the preferential orientation of Cu (111) is obtained onto the Nb or V(110) oriented barrier formed on the amorphous SiOィイD22ィエD2 layer. The interconnect system with these barriers is found to be difficult to form the thin barrier due to the diffusion of barrier materials through the Cu layer and the reaction at the barrier/SiOィイD22ィエD2 interface. For the formation of thin barrier, NbN or VN is applied to the diffusion barrier. Thermally stable thin barrier is obtained in the Cu/VN(10 nm)/SiOィイD22ィエD2 system, however, Cu(111) orientation in the system is inferior to that using a Nb or V barrier. The barrier of Ta-W solid solution maintaining a bcc structure is then applied as a barrier in the above-mentioned system. This system configuration realizes the Cu (111) preferential orientation on the Ta-W (110) barrier, and shows an excellent thermal stability suppressing the diffusion and reaction observed in the system with Nb or V barrier even at a barrier of 20 nm thick. We can achieve the formation of thermally stable thin barrier layer between Cu and SiOィイD22ィエD2 by using Ta-W alloy films with a suitable composition.